Natural carbonates have an enormous importance in the world's economy due to their numerous applications. According to their different uses, such as calcium carbonate in paper and paint industries, the final products have rigorous quality specifications which are difficult to meet.
Thus, efficient, ideally automated, techniques, are required for sorting and separating mineral impurities, which usually comprise varying amounts of dolomite and silica containing rocks or minerals such as silica in the form of flint or quartz, feldspars, amphibolites, mica schists and pegmatite, as disseminations, nodules, layers within the calcium carbonate rock, or as side rocks.
It is the objective in many fields such as in mining or waste industries to have an efficient process of automatically sorting material mixtures.
Automatic particle sorting in this respect means the separation of a bulk flow of particles based on detected particle properties that are measured by electronic sensors such as cameras, X-ray sensors and detection coils.
The suitable technique is chosen according to the particles' characteristics. Thus, there are a number of different sorting techniques, which however mostly have a very limited applicability depending on the specific particle properties. For example, optical sorting requires a sufficient colour contrast of the particles, density separation is only possible at a sufficient difference in the specific density of the particles, and selective mining is mostly inefficient as to time and costs. Where the particles to be sorted have no reliable characteristics allowing for automation, manual sorting has to be applied.
Especially, in the field of mining, the availability of high throughput automatic sorters for coarse and lump sized materials improves the overall efficiency of both mining and milling.
By using automatic rock sorting for pre-concentration, it is possible to mine heterogeneous ore deposits of a lower average grade, but with local sections, bands or veins of high grade. By pre-sorting the ore pieces before grinding, overall milling costs may decrease considerably.
Optical sorters used for minerals processing applications rely on the use of one or more colour line scan cameras and illumination from specially designed light sources. By the camera, a number of distinctive properties can be detected including shape, area, intensity, colour, homogeneity, etc. Typical applications relate to various base metal and precious metal ores, industrial minerals such as limestone and gem stones.
Optical sorters are frequently used for sorting calcium carbonate rocks. However, as mentioned, as soon as the colour contrast is not high enough, separation becomes difficult. For example, flint can be grey, brown or black, but in some quarries also as white as the chalk itself such that an optical sorter cannot remove it from the chalk. Furthermore, even in the case that there is a sufficient colour contrast, the surface of the rocks often has to be wetted and cleaned to enhance the colour contrast and colour stability. In the case of, e.g., chalk however, which is very soft and porous, washing or even wetting is not possible.
Therefore, there is the need to provide sorting techniques other than the usual ones, mainly based on colour contrast, for separating said mineral impurities from calcium carbonate-containing rocks.
X-ray sorters are insensitive for dust, moisture and surface contamination and sorting occurs directly based on the difference of the average atomic number of the rock fragments. Even if there are no visible, electric or magnetic differences, many materials can still be concentrated with X-ray sorting.
X-ray sorters however, up to now, were used especially for sorting scrap metals, building waste, plastics, coals, and metalliferous rocks and minerals, but not for removing said mineral impurities from calcium carbonate rock mainly due to the low differences in mean atomic density between said impurities and calcium carbonate.
For example, WO 2005/065848 A1 relates to a device and method for separating or sorting bulk materials with the aid of a blow-out device provided with blow-out nozzles located on a fall section downstream of a conveyor belt and an X-ray source, computer-controlled evaluating means, and at least one sensor means. The bulk materials mentioned in WO 2005/065848 A1 are ores to be separated, and waste particles, such as glass ceramic from bottle glass, or, generally, different glass types.
GB 2,285,506 also describes a method and apparatus for the classification of matter, based on X-ray radiation. In the method, the particles are irradiated with electromagnetic radiation, typically X-radiation, at respective first and second energy levels. First and second values are derived which are representative of the attenuation of the radiation by each particle. A third value is then derived as the difference between or ratio of the first and second values, and the particles are classified according to whether the third value is indicative of the presence of the particles of a particular substance. In one application of the method, it is used to classify diamondiferous kimberlite into a fraction consisting of kimberlite particles containing diamond inclusions and a fraction consisting of barren kimberlite particles.
U.S. Pat. No. 5,339,962 and U.S. Pat. No. 5,738,224 describe a method of separating materials having different electromagnetic radiation absorption and penetration characteristics. The materials separated by this method are plastic materials being separated from glass materials, metals from non-metals, different plastics from each other. The disclosed method is especially effective at separating items of differing chemical composition such as mixtures containing metals, plastics, textiles, paper, and/or other such waste materials occurring in the municipal solid waste recycling industry and in the secondary materials recycling industries.
WO 2006/094061 A1 and WO 2008/017075 A2 relate to sorting devices including optical sorters, and sorters having an X-ray tube, a dual energy detector array, a microprocessor, and an air ejector array. The device senses the presence of samples in the X-ray sensing region and initiates identifying and sorting the samples. After identifying and classifying the category of a sample, at a specific time, the device activates an array of air ejectors located at specific positions in order to place the sample in the proper collection bin. The materials to be sorted by this device are metals such as lighter weight metals like aluminium and its alloys from heavier weight metals like iron, copper, and zinc and their alloys.
EP 0 064 810 A1 describes an ore sorting apparatus in which the ore to be sorted is selected for sorting according to their absorption of atomic radiation. Ore particles are passed beneath an X-ray tube while being supported on a conveyor belt. X-rays passing through the ore particles impinge on a fluorescent screen. Images formed on the screen are scanned by a scan camera to provide sorting control signals depending on the amount of radiation absorbed by the ore particles. The ores especially examined are tungsten ores, which in particular have proven difficult to be separated using the known detection techniques, but are particularly susceptible to sorting by measurement of X-ray absorptivity under special circumstances.
RU 2 131 780 relates to the beneficiation and sorting of manganese ore including crushing the ore, separating it into fractions according to size, magnetic separation of the fine fraction, and X-ray/radiometric separation of the coarse fraction. Ore with a manganese content of less than 2% goes to dump and ore having more than 2% of manganese is subjected to X-ray/luminescent separation, providing a simplified technological process of winning manganese concentrates from ore.
Thus, there are a number of possibilities how to separate one material from another. However, up to now no efficient technique for sorting and separating mineral impurities from calcium carbonate in calcium carbonate-containing rocks, has been found due to the fact that the present techniques require sufficiently different characteristics such as density and colour of the materials to be sorted, which is problematic regarding many impurities contained in calcium carbonate-containing rocks.
Consequently, there is still a need for alternative techniques for sorting and separating said undesired mineral impurities, also comprising hard, abrasive and/or colouring minerals or rocks, even if there is no distinct colour contrast between the calcium carbonate and said impurities, from the remainder components of the rock.